CN114389707A

CN114389707A - Multi-band linear frequency modulation signal generation method with flexibly selectable frequency

Info

Publication number: CN114389707A
Application number: CN202011111766.9A
Authority: CN
Inventors: 薛寒; 文爱军; 门渊
Original assignee: Xidian University
Current assignee: Xidian University
Priority date: 2020-10-16
Filing date: 2020-10-16
Publication date: 2022-04-22
Anticipated expiration: 2040-10-16
Also published as: CN114389707B

Abstract

The invention discloses a multi-band linear frequency modulation signal generation method with flexibly selectable frequency, and relates to the technical field of microwaves and optical communication. The method is shown in the attached figure 1 of the specification and comprises a laser LD, a dual-polarization binary phase shift keying modulator DP-BPSK, an arbitrary waveform generator AWG, an erbium-doped fiber amplifier EDFA, a polarization controller PC, a polarizer POL and a photoelectric detector PD. The local oscillator signal is subjected to phase modulation through DD-MZM1 of an upper arm of DP-BPSK, and the IF-LFM signal is subjected to carrier suppression double sideband modulation through DD-MZM2 of a lower arm of DP-BPSK; and adjusting the PC to control the phase difference between the two polarization states, and then obtaining the multiband linear frequency modulation signal after POL and photoelectric detection. The invention not only overcomes the electronic bottleneck of the electric domain technology, but also increases the characteristic of flexible frequency tuning; the multi-band signal is generated simultaneously, so that the system has potential application value in multi-band radars and distributed radar systems.

Description

Multi-band linear frequency modulation signal generation method with flexibly selectable frequency

Technical Field

The invention relates to the technical field of optical communication and microwave, and mainly relates to a method for generating linear frequency modulation signals by utilizing a photonics technology.

Background

The increasingly complex detection environment puts higher requirements on radar detection, when a radar system detects a target, the detection distance and the range resolution are indexes which need to be considered at the same time, the emission average power needs to be increased for increasing the detection distance, the pulse width of a detection signal needs to be larger, and the detection signal needs to have a large bandwidth for increasing the range resolution, which is contradictory to general signals, so that pulse compression signals are generated. Chirp signals are one of the microwave waveforms widely used for pulse compression in modern radar systems, and have received increasing attention for decades, and because of their good pulse compression performance, they are not subject to pulse loss due to the velocity of the target, and are widely used. Although the technology for generating chirp signals by using an electronic technology is mature, the development trend of multiple bands and large bandwidth of a future radar is limited by the electronic bottleneck of the current electric domain device, and meanwhile, the development and application scenes of the radar system are also limited by the problems of serious electromagnetic interference, large system volume power consumption and the like.

Due to a series of advantages of microwave photon multiband, large bandwidth, low power consumption, small volume, electromagnetic interference resistance and the like, optical generation of linear frequency modulation signals gradually becomes a research hotspot. By using optical techniques, chirped signals with flexible tuning can be generated over a multi-band range, which can also perform a variety of functions (identification, tracking, mapping, etc.) in a radar detection system.

The most common methods for generating chirp signals reported at present are of two types. One is a spatial-optics based generation scheme and one is a fiber-optics based generation scheme. In the first scheme, although the system is flexible and highly scalable, the free space coupling with the optical fiber will increase the loss and complexity of the system, and the system is usually faced with the problems of large volume and large loss. In the second scheme, no spatial light processing is needed, and the system is more stable; by utilizing a flexible modulation mode and a variable system structure of the electro-optical modulator, linear frequency modulation signals can be simply and effectively obtained, and the time-bandwidth product of the generated signals is greatly improved; however, most of the schemes have single function and can only generate linear frequency modulation signals at a single frequency, and the application of the schemes in a multi-band radar system is difficult.

Disclosure of Invention

In order to solve the problems in the technical background, the invention provides a method for generating a multiband linear frequency modulation signal with flexibly selectable frequency. The method can simultaneously generate multiband linear frequency modulation signals by only using one integrated electro-optical modulator, thereby simplifying the system structure and reducing the transmission loss of the system; the multi-band signal is generated simultaneously, so that the method can be conveniently used in the fields of multi-band radars and the like.

The dual-polarization binary phase shift keying DP-BPSK modulator is adopted to perform phase modulation on optical carriers and inhibit the dual-sideband modulation of the carriers, and polarization states of two paths of optical signals can be orthogonal and multiplexed through a polarization rotator and a polarization beam synthesizer inside the modulator; the photoelectric conversion module adopts the photoelectric detector, and self-heterodyne of local oscillation signals is eliminated under the condition that a balanced photoelectric detector is not needed, so that the photoelectric conversion module is more stable and simpler.

The technical scheme adopted by the invention for solving the technical problems is as follows: the device comprises a laser source LD 1, a dual-polarization binary phase shift keying modulator DP-BPSK 2, a 3 dual-drive Mach-Zehnder modulator DD-MZM, a 490-degree polarization rotator PR, a 5 phase shifter PS, a 6-polarization beam synthesizer PBC, an arbitrary waveform generator AWG 7, an erbium-doped fiber amplifier EDFA, a 9-polarization controller PC, a 10-polarizer POL and a 11-photodetector PD. And the output end of the light source is connected with the input end of the DP-BPSK modulator. The local oscillation signal LO is divided into two paths and loaded at two radio frequency input ends of the DD-MZM1 modulator on the upper arm of the DP-BPSK. An intermediate frequency linear frequency modulation signal IF-LFM signal generated by the AWG is divided into two paths, one path is loaded at one radio frequency input end of the DD-MZM2 modulator of the DP-BPSK lower arm, the other path is loaded at the other radio frequency input end of the DD-MZM2 modulator after being subjected to 180-degree phase shift by the phase shifter, then the output end of the DD-MZM1 is connected with the two input ends of the PBC through PR, the output end of the PBC is connected with the input end of the EDFA, the output end of the EDFA is connected with the PC, the output end of the PC is connected with the POL, the output end of the POL is connected with the input end of the PD, and the output end of the PD can be connected with an oscilloscope or a spectrometer for analysis.

The DP-BPSK is integrated by two parallel dual-drive Mach-Zehnder modulators DD-MZM1 and DD-MZM2, a 90-degree polarization rotator PR and a polarization beam combiner PBC, and each dual-drive Mach-Zehnder modulator is provided with two radio frequency input ends.

The invention comprises the following steps during working:

(1) inputting an optical carrier wave with the wavelength of lambda emitted from a light source into a DP-BPSK modulator;

(2) inputting a local oscillator signal LO to a radio frequency input port of the DP-BPSK modulator;

(3) adjusting bias voltage of a DP-BPSK modulator and power of an input radio frequency signal, outputting a phase modulation signal (optical carrier and positive and negative first-order, positive and negative second-order optical sidebands) by an upper path, inputting an IF-LFM signal by a lower path, outputting a carrier-suppressed double-sideband signal (positive and negative first-order), and mutually orthogonal polarization states of the upper and lower optical signals;

(4) injecting the EDFA amplified signal into a PC to rotate the direction of polarization, set the polarization rotation angle to 45 DEG, and introduce a phase difference between the two polarization directions

The POL is used for synthesizing the polarization multiplexed optical signals into linear polarization signals;

(5) the optical signal output from POL is connected to the input port of PD, and the phase difference introduced by PC is adjusted

After photoelectric detection, a multiband linear frequency modulation signal with selectable frequency is obtained.

The invention provides a method for generating a multiband linear frequency modulation signal with flexibly selectable frequency, which uses a DP-BPSK modulator to respectively realize the phase modulation of an LO signal and the carrier-restraining double-sideband modulation of an LFM signal, and the polarization states of the two paths of optical signals are mutually orthogonal. Inputting the polarization multiplexing signal output by the modulator into PC and POL, setting the polarization rotation angle, and introducing phase difference between the two polarization directions

And then, through photoelectric detection, the generation of multiband linear frequency modulation signals can be realized, and the dual-band, three-band and five-band linear frequency modulation signals can be obtained by setting the phase difference to be 0 degrees, 90 degrees and 45 degrees. Namely, the multi-band chirp signals with flexibly selected frequencies can be obtained under different conditions.

The invention has simple structure, and only adopts one integrated modulator, thereby avoiding phase noise introduced by optical path separation, and ensuring the stability of the system. Furthermore, on the one hand, the entire system does not contain a multi-wavelength laser source or filter, so that a larger operating frequency range of the system is obtained. On the other hand, the signal generator can simultaneously generate multi-band linear frequency modulation signals with flexibly selectable frequencies, so that the signal generator can find suitable application in a distributed coherent radar system.

Drawings

Fig. 1 is a schematic diagram of an optical generator of a frequency selectable multi-band chirp signal.

Figure 2 shows (a) the corresponding spectrum of the DP-BPSK output signal and (b) the corresponding spectrum of the signal after POL,

is the phase shift introduced by the POL.

Fig. 3 is a graph of (a) the spectrum of the LO signal and (b) the spectrum of the IF-LFM signal.

Fig. 4(a) is an electric spectrum of LFM signals of five frequency bands (2GHz, 6.5GHz, 11GHz, 15.5GHz, 20GHz) generated with a phase difference set at 45 °, fig. 4(b) is an electric spectrum of LFM signals of two frequency bands (6.5GHz, 11GHz) generated with a phase difference set at 0 °, and fig. 4(c) is an electric spectrum of LFM signals of three frequency bands (2GHz, 15.5GHz, 20GHz) generated with a phase difference set at 90 °.

Fig. 5 is a time-frequency diagram of a multiband LFM signal obtained when the phase difference between two polarization directions is set to (a)45 °, (b)0 °, and (c)90 °, respectively.

Fig. 6(a), (b), (c), (d), (e) are waveform diagrams, (ii) time-frequency diagrams and (iii) autocorrelation result diagrams of the LFM signals of 2GHz, 6.5GHz, 11GHz, 15.5GHz and 20GHz bands, respectively.

Fig. 7(a) is an electric spectrum diagram of LFM signals of five frequency bands (1GHz, 3.5GHz, 6GHz, 8.5GHz, 11GHz) generated with a phase difference set at 45 °, fig. 7(b) is an electric spectrum diagram of LFM signals of two frequency bands (3.5GHz, 6GHz) generated with a phase difference set at 0 °, and fig. 7(c) is an electric spectrum diagram of LFM signals of three frequency bands (1GHz, 8.5GHz, 11GHz) generated with a phase difference set at 90 °.

Fig. 8 is a time-frequency diagram of a multiband LFM signal obtained when the phase difference between two polarization directions is set to (a)45 °, (b)0 °, and (c)90 °, respectively.

Fig. 9(a), (b), (c), (d), (e) are waveform diagrams, (ii) time-frequency diagrams, and (iii) autocorrelation result diagrams of the LFM signals in the 1GHz, 3.5GHz, 6GHz, 8.5GHz, and 11GHz bands, respectively.

Detailed Description

The embodiments of the present invention will be described in detail below with reference to the accompanying drawings: the present embodiment is implemented on the premise of the technical solution of the present invention, and a detailed implementation manner and a specific operation flow are given, but the scope of the present invention is not limited to the following embodiments.

Fig. 1 is a schematic diagram of a multi-band chirp signal generation method with flexibly selectable frequencies. The device comprises a laser source LD 1, a dual-polarization binary phase shift keying modulator DP-BPSK 2, a 3 dual-drive Mach-Zehnder modulator DD-MZM, a 490-degree polarization rotator PR, a 5 phase shifter PS, a 6-polarization beam combiner PBC, an arbitrary waveform generator AWG 7, an erbium-doped fiber amplifier EDFA, a 9-polarization controller PC, a 10-polarizer POL and a 11-photodetector PD. DP-BPSK realizes polarization multiplexing of two optical signals, and upper arm DD-MZM1 works at maximum transmission point MATP to perform phase modulation on local oscillation signal LO. The DD-MZM2 modulator of the lower arm works at the minimum transmission point MITP, and carries out the carrier suppression double sideband modulation on the IF-LFM signal generated by the AWG, and FIG. 2(a) is a spectrum diagram of the orthogonal polarization multiplexing signal output by the DP-BPSK modulator. The amplified signal is then injected into the PC to rotate the direction of the polarization state and introduce a phase difference between the two polarization directions. After the PC, the POL is used to synthesize the polarization-multiplexed optical signals into linearly polarized optical signals, and FIG. 2(b) shows that the output optical signals of the POL can be connected to an oscilloscope or a spectrometer for analysis after being subjected to PD photoelectric conversion.

In this example, the method specifically includes the following steps:

the method comprises the following steps: the light source generates a continuous light wave with the working wavelength of 1550.1nm and the power of 16dBm, and the continuous light wave is input into the DP-BPSK modulator as a carrier wave;

step two: the analog signal generator outputs a sinusoidal local oscillator signal with a frequency of 9GHz and a power of 23dBm, then the LO signal is divided into two parts and directly injected into two RF ports of an upper arm DD-MZM1 of the DP-BPSK modulator, and the sub-modulator bias voltage is adjusted to work at the maximum transmission point MATP to realize phase modulation. The AWG generates an IF-LFM signal with a carrier frequency of 2GHz and a bandwidth of 0.5GHz, and the power of the IF-LFM signal is-2 dBm. The output IF-LFM signals are directly injected into two RF ports of the DD-MZM2 by a pair of differential output ports, and the bias voltage of the sub-modulator is adjusted to work at the minimum transmission point MITP for double-sideband modulation of carrier suppression;

step three: the optical signal output by the modulator is amplified by the EDFA, and the noise coefficient is 4.5 dB. The EDFA works in an automatic power control mode, the output power is 10.5dBm, and the EDFA is used for compensating the loss of an optical link;

step four: the amplified signal is injected into the PC to rotate the direction of the polarization state and introduce phase differences of 0 °, 45 ° and 90 ° between the two polarization directions, respectively. After the PC, synthesizing the polarization-multiplexed optical signal into a linearly polarized optical signal using POL;

step five: the linearly polarized light signal is injected into the PD (responsivity 0.74A/W) to perform photoelectric conversion. Observing the electric spectrogram by using a spectrum analyzer, recording a waveform in an oscilloscope, and performing processing and autocorrelation calculation by using MATLAB;

step six: repeating the second step to the fifth step, respectively changing the carrier frequencies of the IF-LFM signal and the LO signal into 1GHz and 5GHz, and verifying the carrier frequency tunability of the generated signals;

fig. 3(a) is a spectrum diagram of LO phase modulation of a local oscillator signal LO output from an upper arm of DP-BPSK, and fig. 3(b) is a spectrum of an IF-LFM signal output from a lower arm of DP-BPSK; fig. 4(a), (b), (C) are diagrams of LFM signal spectrograms obtained by setting the phase difference between two polarization states equal to 45 °, 0 ° and 90 ° for all five, two and three bands, respectively, to generate signals covering a plurality of frequency bands, such as the S (2GHz), C (6.5GHz), X (11GHz), Ku (15.5GHz) and Ka (20GHz) frequency bands; fig. 5(a), (b) and (C) show the LFM signals of five, two and three bands obtained after short-time fourier transform, respectively, positive chirp signals at S (2GHz), X (11GHz) and Ka (20GHz) bands, and negative chirp signals at C (6.5GHz) and Ku (15.5GHz) bands, with the selected band power being about 20dB higher than the rejected band, demonstrating the good band selection capability of the obtained signals; fig. 6(a) - (e) are graphs of (i) waveforms, (ii) time-frequency diagrams, and (iii) autocorrelation results of the LFM signals in the 2GHz-20GHz band. The autocorrelation peaks of the resulting signal are shown in fig. 6(iii) to assess the pulse compression capability of the obtained signal. Each autocorrelation result shows a full width at half maximum (FWHM) of 0.72ns, 0.73ns, 0.72ns, 0.75ns, and 0.73ns, respectively, corresponding to Pulse Compression Ratios (PCRs)1389, 1370, 1389, 1333, and 1370, respectively. In addition, the peak sideband rejection ratio (PSR) of each autocorrelation peak was 7.29dB, 7.52dB, 7.47dB, 7.32dB, and 7.34 dB. The autocorrelation result shows that the obtained signal has good pulse compression capability. Fig. 7(a) (b) (c) in order to set the phase difference between the two polarization states equal to 45 °, 0 °, and 90 ° to obtain the LFM signals of all five bands, two bands, and three bands, respectively, the carrier frequencies of the IF-LFM signal and the LO signal are changed to 1GHz and 5GHz, respectively, the frequency bands in which the LFM signals are generated are 1GHz, 3.5GHz, 6GHz, 8.5GHz, and 11GHz, respectively, the tunability of the carrier frequencies of the generated signals is verified; fig. 8(a), (b), and (c) are LFM signals of five bands, two bands, and three bands, respectively, obtained after short-time fourier transform, the LFM signals being generated at 1.5GHz, 3.5GHz, 6GHz, 8.5GHz, and 11GHz, respectively, and it was found that the obtained signals have good band selection capability; fig. 9 shows (a) - (e) graphs of the waveforms, (ii) time-frequency graphs, and (iii) autocorrelation results of the generated signals of the LFM signals corresponding to the 1GHZ-11GHZ band, where (a) - (e) each autocorrelation result has a FWHM of 0.73ns, 0.74ns, 0.72ns, 0.75ns, and 0.74ns, corresponding to PCRs of 1370, 1351, 1389, 1333, and 1351, respectively. Further, the PSR of each autocorrelation peak is 7.22dB, 7.46dB, 7.51dB, 7.35dB, and 7.32dB, respectively. Again showing good pulse compression capability of the resulting signal.

In the scheme, only one integrated modulator is used, so that the influence of introduced phase noise on signals is avoided, and the stability of the system is enhanced. On the one hand, the entire system does not contain a multi-wavelength laser source or filter, thereby allowing a larger operating frequency range for the system. On the other hand, the signal generation method can simultaneously generate the multiband LFM signals with flexibly selected frequencies, so that the application range is wider, and the method has application value in the fields of multiband radars, distributed coherent radar systems and the like.

In conclusion, the above-described embodiments are merely examples of the present invention and are not intended to limit the scope of the present invention, it should be noted that, for those skilled in the art, many equivalent modifications and substitutions can be made on the disclosure of the present invention, such as using separate devices to implement the function of the DP-BPSK modulator. In addition, changing the wavelength and power of the input optical carrier, changing the power of the local oscillator signal, changing the power of the IF-LFM signal, etc. are the protection scope of the present invention.

Claims

1. A multi-band linear frequency modulation signal generation method with flexibly selectable frequency comprises a laser LD, a dual-polarization binary phase shift keying modulator DP-BPSK, an arbitrary waveform generator AWG, a phase shifter PS, an erbium-doped fiber amplifier EDFA, a polarization controller PC, a polarizer POL and a photoelectric detector PD, wherein the DP-BPSK is internally integrated by a Y-type optical splitter, two parallel dual-drive Mach-Zehnder modulators DD-MZM1 and DD-MZM2, a 90-degree polarization rotator PR and a polarization beam combiner PBC; the method is characterized in that: the optical carrier output by LD enters DP-BPSK, the local oscillator signal LO is divided into two paths, and respectively input into two RF driving ports of DD-MZM1, the bias voltage is adjusted, so that DD-MZM1 works at maximum point MATP, phase modulation is realized, the intermediate frequency linear frequency modulation signal IF-LFM generated by AWG is divided into two paths, and respectively drives two RF ports of DD-MZM2, one path of the light signals is shifted by 180 degrees through PS, the bias voltage is adjusted, the DD-MZM2 works at the minimum point MITP, the carrier double-sideband modulation is restrained, the polarization states of the two paths of light signals output by the DD-MZM1 and the DD-MZM2 are mutually orthogonal after passing through a 90-degree polarization rotator, then the two paths of light signals are output after passing through PBC, the output end of DP-BPSK is connected with EDFA, the output end of EDFA is connected with PC, the polarization rotation angle is set, and the phase difference is introduced between the two polarization directions.

Then POL is connected, the polarization multiplexing optical signals are synthesized into linearly polarized optical signals, then PD is connected, after photoelectric detection, multiband linear frequency modulation signals can be obtained, and phase difference is enabled to be achieved by adjusting PC

Respectively at 0 deg., 90 deg. and 45 deg. to obtain dual-band, three-band and five-band chirp signals, i.e. adjusting PC to change phase difference

The multi-band linear frequency modulation signal with flexibly selected frequency can be obtained.